The present invention is directed to a combined adhesion promotion and direct metallization process for non-conductive substrates. More specifically, the present invention is directed to a combined adhesion promotion and direct metallization process for non-conductive substrates employing a cobalt etch to both improve adhesion between non-conductive surfaces and a plated metal and to improve the direct metallization process.
Non-conductive surfaces may be metallized by a sequence of steps consisting of catalysis of the surface of the non-conductor followed by contact of the catalyzed surface with an electroless plating solution that deposits metal over the catalyzed surface in the absence of an external source of electricity. Following electroless metal deposition, the electroless metal deposit is optionally enhanced by electrodeposition of a metal coating to a desired full thickness.
Catalyst compositions used for electroless metal plating are known in the art and disclosed in numerous publications such as in U.S. Pat. No. 4,734,299. The catalysts of the patent consist of an aqueous suspension of a tin-noble or precious (catalytic) metal colloids such as tin/palladium. Electroless plating solutions are aqueous solutions containing dissolved metal and reducing agent in solution. However, the presence of dissolved metal and reducing agent together in solution results in solution instability and plate out of metal in contact with a catalytic metal-tin catalyst on the walls of containers. To prevent the plate out of metal on the walls of containers, stabilizers such as toxic cyanide compounds are included in the solutions. Such compounds impose a hazard to workers in the art. Also environmentally undesirable complexing agents and chelating agents such as ethylenediamine-tetraacetic acid (EDTA) are employed to keep copper in solution in strongly alkaline media. Additionally, reducing agents such as formaldehyde are known carcinogens and are desirably substituted with boron or another type of reducing agent. However, substitutes such as the boron reducing agents are costly in contrast to formaldehyde which is considerably more cost effective.
Electroless plating processes involve a relatively large number of steps resulting in undesirably high processing times and water consumption. In addition to the problems associated with the combination of dissolved metal and reducing agent in the same solution, and the large number of steps, the use of noble or precious metals such as palladium in metal colloids adds to the cost of the electroless processes. Such disadvantages have spurred the search for methods that provide viable alternatives to electroless depositon methods.
EP 0 913,498 A1 provides an improvement over the electroless plating method of U.S. Pat. No. 4,734,299. EP 0 913,498 A1 discloses an electroless plating method where costly noble metals such as palladium need not be employed in catalytic metal colloids. Thus, the process is less costly. EP 0 913,498 A1 employs metal activators such as silver, cobalt, ruthenium, cerium, iron, manganese, nickel, rhodium and vanadium. The metal activators are applied to a non-conductive surface as salts such as sulfate salts in aqueous sulfuric acid solution, nitrate salts in nitric acid solution, fluoroborate salts in aqueous fluoroboric acid solution and the like. The metal activators provide a roughened surface on the non-conductive substrate without the need for harsh conditioning agents such as chromic acid and permanganate. In use, the metal activators are oxidized to a higher state (e.g. to Ag2+ or to Co3+) electrochemically. Workers in the art believe that the oxidized metal activator generates reactive hydroxyl species from water, such as hydroxyl radicals. The hydroxyl species are very reactive and attack the non-conductive surfaces, particularly organic polymers, to provide an ideal pitted or roughened morphology deemed desirable for subsequent metal adhesion. Following surface texturing, the non-conductive substrate is immersed in a reducing agent pre-dip solution. The substrate is then immediately plated by immersion into an electroless metal plating solution such as a copper bath without any further process steps. Examples of reducing agents employed in the process include formaldehyde, boron based reagents such as dimethylamineborane, boron hydride or salts thereof (e.g. sodium borohydride), sodium dithionite, or sodium hypophosphite.
Another advantage of the method is that the metal activators, such as silver and cobalt, may be regenerated by electrochemical methods thus reducing problems encountered with waste treatment. As mentioned above, EP 0 913,498 A1 also has the advantage that the process disclosed therein does not employ a chromic acid etch as many electroless plating processes utilize. An example of a chromic acid etch may be found in U.S. Pat. No. 4,457,951. Chromic acid etch methods are undesirable because chromic acid may be a carcinogen, and also presents various waste treatment, disposal problems and environmental concerns. Accordingly, there is a great interest in the development of alternative methods of metallization that do not utilize chromic acid.
Although the electroless plating method disclosed in the EP 0 913,498 A1 patent is a very good electroless plating method for the reasons just discussed, the method still employs many reducing agents that are undesirable for the reasons discussed earlier, and an electroless plating step. Electroless plating steps increase the time over which the plating process is performed in contrast to direct electroplating, thus reducing the efficiency of the plating process. Other disadvantages include more complex and expensive waste treatment procedures, greater environmental impact and greater water consumption than direct metallization processes.
Attempts have been made to avoid electroless plating processes by a direct metal plating process whereby a metal is deposited directly over a treated non-conductive surface. U.S. Pat. No. 5,007,990 discloses a method for direct electroplating the surface of a non-conductor and to articles manufactured by the process. The invention disclosed in the patent is based upon a combination of discoveries. One discovery was that chalcogenide films of metals that function as electroless deposition catalysts may be electroplated directly without requiring an intermediate electroless coating. Another discovery of the invention was that many of such chalcogenide films are insoluble and unaffected by treatment chemicals used for plating of plastics and circuit board fabrication and therefore, the process of the invention was suitable for the formation of printed circuits using pattern plating processes.
In the process of U.S. Pat. No. 5,007,990, the electroless metal plating catalysts are believed to be colloids formed by the reaction of stannous salt and a noble metal salt such as palladium with a chalcogenide. Other metals that may be employed include copper, nickel, cobalt and the like. However such non-noble metals are lesser preferred than palladium. Non-noble metals such as cobalt are less catalytic than palladium. Solutions for the formation of chalcogenide films may be thio compounds and divalent sulfide solutions. Alkaline earth metal sulfide salts provide the source of the sulfide. Such salts include sodium, potassium and lithium sulfides. Accordingly, preferred coatings for direct electroplating are believed to be films of mixed sulfide or palladium and tin.
Although U.S. Pat. No. 5,007,990 is an improvement in plating a non-conductive substrate by eliminating the electroless step from the process, the process preferably employs costly palladium metal over less costly metals such as copper, nickel, or cobalt. Additionally, when employing the process in the manufacture of printed circuit boards, board through-holes are desmeared by sulfuric acid, chromic acid or plasma etching or etchback of the holes with chromic acid prior to applying the process of the ""990 patent. As discussed above, chromic acid steps are undesirable. Also use of sulfuric acid in its etching concentrations may present hazards to workers. Sulfuric acid may cause severe burns to workers and may cause corrosion of metal parts on printed circuit boards.
WO 00/29646 discloses a method of treating a non-conductive surface for metallization. The method involves etching a non-conductive surface, such as a plastic surface, with an oxidizing agent to roughen the non-conductive surface. Such etching materials include permanganate, chromic acid, hydrogen peroxide, and the like. After the plastic surface is roughened, the surface is treated with a cobalt salt, silver salt, tin salt or lead salt. Preferably, following treatment of the plastic with the metal salt, the plastic is treated with an aqueous alkaline solution at a pH of from 9 to 14. Alternatively, the plastic can be treated with an acid solution. Such treatment is employed when cobalt sulfate is used as the metal salt as taught in the examples at the end of the patent. The plastic surface then is treated with an alkali metal sulfide such as a monosulfide, disulfide, tetrasulfide or polysulfide. For example, sodium sulfide (Na2S) may be employed. After applying the sulfide solution, the plastic is plated by a direct plating method.
Although WO 00/29646 eliminates the use of a costly metal as palladium, as well as an electroless plating step, the method employs an etching method employing chromic acid, permanganate or other acid or highly basic oxidizing agents that may present a hazard to workers or to the environment. Further, when cobalt salts are employed, as disclosed in the examples in the patent, an additional step of treating the plastic with an acid or alkaline dip is employed. Accordingly, there is still a need for a more efficient direct metallization method that eliminates both harsh oxidation steps to roughen non-conductive substrates, the use of costly noble metal catalysts, and electroless plating steps.
The present invention is directed to a combined process for improving the adhesion of a metal to a non-conductive substrate and direct metallization of the non-conductive substrate by texturing the non-conductive substrate with a cobalt etch and treating the textured non-conductive substrate with a sulfide to provide a conductive surface or layer. Advantageously, etching a non-conductive substrate with a cobalt etch texturizes the non-conductive substrate to provide a sufficiently roughened surface such that a metal deposited on the textured surface forms a bond of high integrity with the substrate. Further, the treatment of the cobalt etched non-conductive substrate with a sulfide provides a conductive cobalt sulfide species such that a metal can be directly plated on a now conductive surface of the non-conductive substrate.
Advantageously, the method of the present invention provides for an efficient method of texturing a non-conductive substrate and activating the non-conductive substrate to receive a metal deposit. The method of the present invention eliminates the steps of electroless plating as well as texturing the non-conductive substrate with an oxidizer such as hydrogen peroxide, chromic acid, permanganate and the like. The cobalt etch is sufficient to provide a textured surface on the non-conductive substrate without the use of an oxidizer. The method of the present invention also eliminates the use of undesirable reducing agents such as formaldehyde and boron compounds, and costly noble metal catalysts such as tin/palladium catalysts. Thus the method of the present invention not only provides a shorter and more efficient direct metallization process, but the process also eliminates the numerous problems associated with other metal plating methods.
Another advantage of the method of the present invention is that the method may be employed in both horizontal and vertical methods of manufacturing printed circuit and wiring boards. The method of the present invention is short, thus being highly suitable for horizontal circuit board manufacturing processes.
Horizontal methods are a highly desirable methods of preparing printed wiring boards because the process is more rapid than the vertical method, thus out-put is greater. Horizontal methods are more costly than vertical methods, because of the equipment employed, thus reducing the time of operation is highly desirable. The reduced number of method steps of the present invention to both texture and provide a conductive surface on the non-conductive substrate is highly desirable for horizontal methods.
A primary objective of the present invention is to provide for a method of texturing and providing a conductive surface on a non-conductive substrate such that the substrate can be directly metallized.
Another objective of the present invention is to provide a method for texturing a non-conductive surface without the use of oxidizing agents.
An additional objective of the present invention is to provide a method of direct metallization without an electroless plating step.
Still yet an additional objective of the present invention is to provide a method of direct metallization that does not employ noble metal catalysts.
Further objectives and advantages of the present invention may be determined by a person of skill in the art after reading the detailed description of the invention and the appended claims.